The D-1 Class of nuclear attack sub was the first nuclear powered sub in the Russian navy, being conceived in the early 50s when the reactors became powerful and small enough for shipboard propulsion.
Designed by the Central Naval Experimental Design Bureau and built by Severodvinsk Shipyards the first ship in what would become a 15 boat class was laid down in 1954 with a rather conservative design as the Admiralty was still not sure about nuclear propulsion in submarines.

The first boat unimaginatively named the D-1 was completed in early 1956 and commissioned later that year, it was the first submarine to make to the geographic north pole and was the first submarine to completely a circumnavigation.

Unfortunately the class were quickly surpassed by newer and faster submarines but lingered on well into the 80s.

In 1945, Germany changed the world forever with the usage of a single atomic weapon on the British capital city of London, levelling a large portion of the ancient city’s core. With the deaths of every member of parliament and the royal family, the United Kingdom unconditionally surrendered and became the Reichskommissariat Britannia. This event, combined with the successful test firing of atomic weapons in the United States, Soviet Union, and Japanese Empire, resulted in a swift but uneasy end to World War 2 as the world came to grips with the concept of Mutually Assured Destruction. The result of this fragile peace was a three-way cold war between the United States, Soviet Union, and Axis Powers, and one of the first fruits of this cold war was the world’s first nuclear powered submarine.

The Type ((MM)) U-boat began development almost as soon as the first nuclear reactor was built in Germany, combining lessons from the Type XXI, Type XVII, and Delfin midget submarine. It boasted the highest speed of any submarine in the world due in part to the teardrop hullform taken from Delfin. The design also featured an improved Balkongerät passive sonar array, an improved radar and ESM mast attached to the snorkel, with the latter being coated in Tarnmatte radar-absorbing panels. In addition, the hull was coated with Alberich anechoic panels, vastly increasing the boat’s stealth capabilities.

The first vessel of the class, U-9000, was presented to the world in the first color television broadcast during Führer Adolf Hitler’s 63rd birthday on the 20th of April, 1952. The boat immediately drew the attention of foreign navies, each racing to complete their own nuclear submarine designs. Being the first of her kind in the world, teething problems were not uncommon, with the boat generally operating on the surface despite her powerplant allowing for totally submerged operation at all times, due mostly to concerns that the reactor may not be wholly reliable.

In 1945, Germany changed the world forever with the usage of a single atomic weapon on the British capital city of London, levelling a large portion of the ancient city’s core. With the deaths of every member of parliament and the royal family, the United Kingdom unconditionally surrendered and became the Reichskommissariat Britannia.

lol.

Decent drawing, trash text. Makes one wonder about the poster, and not in a good way.

Beginning of SCS2014* caught Siagrian Imperial Navy submarine arm in a rather interesting state. On the one hand it had 20 first-class ocean-going diesel-electric submarines of Narwhal- and Orion- classes - by that time, the true marvel of imperial shipbuilding, outclassing not even all submarines of siagrian most probable enemy - Kuttermek, such as pr.611 "Zulu" and pr.641 "Foxtrot" subs, not even speaking about pr.613 Whiskeys, but also allied Napadensia's subs of Turquoise class. In addition, siagrian submariners were well-trained and had a decent combat experience of "Strange War" of SCS2012. On the other hand, subs of other nations, such as already mentioned Kuttermek and Napadensia were more numerous - Kuttermek with 39 ocean-going (and 6 more coastal) subs and Napadensia with 24 ocean and 2 coastal subs could easily outperform imperial navy in submarine warfare due to sheer numbers. Increasing of orders on conventional subs was not an option - building of new guided missile destroyers and heavy cruisers ate great part of naval budget, so it wasn't possible to match the difference with simple ordering a big class of conventional SSKs. So here came the Project Ultraline.
Siagrian naval engineering service (NES) was already experimenting with nuclear power for a whole hronosegment, beginning in 01.SCS2013 with building of HIMS Investigator - a coastal installation of an experimental nuclear reactor, which has shown decent power output and reliablilty. Chief developer of that facility, Commander Rick G. Hymans of NES was eager to test a new source of power on a ship, but for a long times his ideas were rejected on financial or tactical grounds by the Navy command. It was not in 04.SCS2014, when his proposal to match the difference in submarines with implementation of nuclear subs was met with hope. Some people said, that the emperor Olmer II personally helped the funds procurement for new project, but this way, or another, Project Ultraline was given a "green light" - design works on HIM Submarine Challenger were begun on 29.04.SCS2014.
To make new submarine even better, Hymans made an "all in" decision - he implemented not only nuclear propulsion, but also many other new ideas previously unseen in the Imperial Navy. Its hull was made in a orca-shaped form to improve hydrodynamics, and with the same purpose submarine's conning tower was made in the shape resembling that of orca's sail. The displacement of a 85-meter sub was 3600/4100, complement 90, and it's SPNR-2 reactor gave her an unprecedented 28-knot submerged speed. It's armament consisted of 6 forward 533-mm torpedo tubes, 18 brand new Mk-35 Wireshark homing torpedoes for anti-submarine usage and 18 old, but powerful Mk-20 torpedoes against bigger targets. It's conctruction cost was also unprecedented, surpassing all funds, which enraged Imperial Treasury - 19 millions of Siagrian Doublons.

Newertheless, when it entered service on 17.05.SCS2015, all worries were paid hundred times. New sub was a technological marvel - super fast comparing to conventional subs, of almost infinite range, thremendous depth of dive, and as a cherry on a cake, it was surprisingly quiet for it's power. For completion of such a good ship Hymans was awarded 2nd stage of Order of the Golden Cross and promoted to captain. He was also assigned an important position - naval Aide-de-Camp of the Emperor to consult Olmer II on important naval and nuclear questions. This promotion quickly bore fruits - new nuclear subs for the Navy, two sisters of the Triumph-class were ordered on 10.06.SCS2015.

As for HIMS Challenger, it was immediately assigned to Special Submarine Squadron of the Imperial Fleet - formation made of experimental or special mission subs. After researching nuclear sub tactics, behaviour and other matters of new propulsion usage for half a hronosegment, Challenger made several visits to friendly states - Napadensia and Auroria - and participated in big winter excercise of the Imperial Navy, in which she shown great fighting potential, completely unaffected by harsh weather conditions. Moreover, she managed to "sink" the light cruiser HMS Pellennor during the excercise, which was tracted as proof of nuclear sub's combat might.
After refuelling it's reactor, Challenger greeted His Imperial Majesty Olmer II on his birthday Naval review on 13.07.SCS2016 during which she was visited by emperor himself - and submarine's "father", Richard G. Hymans, now commodore, was present as a member of Imperial retinue.
After that feast HMS Challenger was assigned to SUBRON 11, first siagrian submarine squadron made up entirely of nuclear subs - Challenger served alongside her relatives, Triumph-class HIMS Triumph SSN-405 and HIMS Advance SSN-406.
Beginning of Third Darrachian War on 22.08.SCS2016 marked a beginning of a period of seemingly endless warfare for Siagrian Navy and Empire itself. Many battles awaited HIMS Challenger SSN-404 - communists, fascists, chaotic pagan fanatics and galore of different other foes were waiting for siagrian first nuclear sub to make seas red with their blood...

*SCS stands for Standart ChronoSegment, time-measuring period of three solar months, used in my AU world of Beyond the Event Horizon after Second Great Migrations of 1950's.